The peptide hormone prolactin (Prl) exerts a profound effect on both cellular proliferation and differentiation in a variety of tissues, but its role as a immunomodulator has only recently received attention. Its potential importance is supported by the observation that the Prl receptor belongs to the newly identified cytokine receptor superfamily. As a model system for studying how Prl modulates the proliferative response of T lymphocytes, we are using the rat Nb2 T lymphoma cells which require Prl for growth. We isolated a cDNA from Nb2 T cells which showed rapid and dramatic induction by Prl, and found that this cDNA encoded the transcription factor interferon regulatory factor-1 (IRF-1), previously shown to be important in regulating interferon (IFN) gene expression in fibroblasts. We further demonstrated that IRF-1 is induced by the T cell mitogen Con A in murine splenocytes and thymocytes. Our studies suggest that IRF-1 may be a growth-related master switch gene which plays an important regulatory role in T cell activation and proliferation. Very little is known about what regulates IRF-1 gene expression in T cells, what the IRF-1 transcription factor regulates in turn, and how IRF-1 promotes Prl-stimulated cell proliferation. The primary objective of this proposal is to elucidate the role of IRF-1 in T cell activation mediated by Prl and mitogens. Studies are proposed to: 1) analyze the promoter region of the IRF-1 gene, to identify regulatory sequences mediating Prl stimulation, and to characterize the nuclear proteins interacting with the Prl responsive DNA elements; 2) analyze the biochemical properties of IRF-1 protein, with emphasis on posttranslational modification (phosphorylation) as another level of regulation by Prl; and 3) investigate the functional role of IRF-1 in T cell activation and proliferation, by altering IRF-1 levels and examining its effect on cell growth. These studies represent a comprehensive molecular and immunological approach to defining the structure and function relationship of IRF-1 and elucidating the immunoregulatory properties of Prl. Understanding how Prl regulates T lymphocyte function and proliferation through regulation of IRF-1 activation in T cells, will provide new insights into the intricate regulatory circuits between the neuroendocrine and immune systems.